i haven't gotten this achievement yet but had a chance earlier in my playthrough, i kept shooting the glass with the sniper rifle, eventually it shattered, i then turned away for a bit to take care of another enemy and i noticed that the shield regenerates on the atlas mech. right after i was able to shoot the enemy inside without the mech. blowing up, unfortunately someone else jumped in and i was unable to shoot the enemy out of it. i think a good strategy is to shatter the glass, let the shields regenerate, then take out the enemy and jump in asap.